Flash pyrolysis of coals - behaviour of three coals in a 20 kg h-1 fluidized-bed pyrolyser

Flash pyrolysis of coals: behaviour of three coals in a 20 kg h-l fluidized-bed pyrolyser
James H. Edwards and Ian W. Smith
CSIRO Division of Process Technology, P.O. Box 136, North Ryde, NSW, Australia (Received August 1979; revised 11 April 19801 2113
Flash pyrolysis of Loy Yang brown coal, and Liddell and Millmerran bituminous coals has been studied using a fluidized-bed reactor with a nominal throughput of 20 kg h-‘. The apparatus and its performance are described. The yields of tar and hydrocarbon gasesare reported for each coal in relation to pyrolysis temperature, as also are analytical data on the pyrolysis products. The peak tar yields for the dry, ash-free Loy Yang and Millmerran coals were respectively 23% wt/wt (at ~580’C) and 35% wt/wt (at =6OO”C). The tar yield from Liddell coal was 31% wt/wt at ~580’C. Hydrocarbon gaseswere produced in notable quantities during flash pyrolysis; e.g. Millmerran coal at 810°C gave 6% wt/wt (daf) methane, 0.9% wt/wt ethane, 6% wt/wt ethylene, and 2.5% wt/wt propylene. The atomic H/C ratios and the absolute levels of hydrogen in product tars and chars decreased steadily with increasing pyrolysis temperature.
674
FUEL,
1980,
Vol 59, October
Flash pyrolysis
of coals in a 20 kg h-’
reactor:
J. H. Edwards
and I. W. Smith
F
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Figure 7
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GAS
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Australia has a strong and growing need for liquid fuels’ and a rapidly diminishing supply of indigenous oil’,‘, but abundant coal reserves*. The CSIRO is therefore carrying out a programme of research into the production of oil from coal. The main emphasis of this programme at present is on the flash pyrolysis technique, the object being to determine the technical and economic feasibility of the technique for converting coal to oil. Details of the project have been given elsewhere3,4. In brief, flash pyrolysis of coal gives char, gas, and an appreciable quantity of tar which is subsequently hydrogenated to a range of liquid fuels. In the course of the project various pyrolysers have been operated. In particular a 1 g h-l fluidized-bed pyrolyser has been used to determine the flash pyrolysis behaviour of a range of Australian and other coals5.6. The data have given insight into the chemistry of flash pyrolysis, and have provided the basis for calculations of the performance of conceptual oil-from-coal processes3. Rapid, or flash, pyrolysis of coal has been carried out by other workers, using various types of reactors including fluidized beds. Reviews of this work are available7.8. The relation between process conditions and tar yields has been considered by Sitnai and Smith’. However, little information is available on the thermal decomposition of pulverized coal in fluidized beds of coarse sand, with short (i.e. = 1 s) vapour contact times - the conditions of the experiments described here. This paper describes experiments with a fluidized-bed pyrolyser with a nominal coal input of 20 kg h-‘. The objectives were to check the effect of scale-up on yields found for the I g h-’ pyrolyser5,“, and to provide adequate quantities of tar and char for further testing. Results from the pyrolysis of three coals, Loy Yang brown coal (Victoria), and bituminous coals from Liddell (New South Wales) and Millmerran (Queensland), are reported.
0016-2361 @ 1980 I801 100694-0752.00 I PC Business Press
The data include the yield of tar and hydrocarbon gases in relation to unit quantity of dry, ash-free (daf) coal over the temperature range 460-810°C.
EXPERIMENTAL
Reuction system
The reaction system is illustrated in Figure 1. It has three main sections: a coal feeder, the reactor containing a fluidized bed of sand, and a product recovery system; each appropriately instrumented and equipped for safety, control and monitoring. The coal feeder vessel (0.51 m diameter, 1.63 m high) holds 200 kg of dried coal in the fluidized state. It is mounted on load cells, the output from which is recorded. The coal is fluidized with metered streams of nitrogen,chnique of Huff et al. lo. The vessel is pressurized by throttling the vent nitrogen, and coal is fed to the reactor through 3 mm bore lines. The coal offtake system is based on that developed by Hamor and Smith (Figure 3~ of Reference 11); two offtake lines are used, one is opened automatically for 150 s whilst the other is shut for cleaning by automatic purges. The out-ofphase on-off cycling and cleaning of the lines ensures steady coal feeding. The hot gas for fluidizing the sand bed in the reactor and transporting the reaction products is supplied in one of two ways. It can be generated by combustion of LPG and air in a burner chamber, with addition of cold nitrogen to the combustion products if necessary for temperature moderation, or the gas can consist mainly of nitrogen which is brought to temperature in an electrical preheater, and further heated, if necessary, by injection of combustion products from an acetylene-air burner. Both the acetylene and LPG burners are operated slightly gas-